The present invention relates generally to current limiting circuits and, more particularly, to a current limiter suitable for use with a Subscriber Loop Interface Circuit (SLIC) or the like for limiting current flow. In particular, the current limiter of the subject invention is utilized in combination with a SLIC to limit the subscriber loop current to a maximum, predetermined value.
Hybrid circuits, or SLICs, which are well known, provide signal conversion between a balanced two-wire bidirectional transmission path and a pair of unbalanced, unidirectional signal paths. SLICs are typically employed in telephony systems to provide signal interface between a telephone carrier channel having unidirectional transmission and receive signal paths to a balanced subscriber loop comprising a telephone instrument. Such SLICs or hybrid circuits have been disclosed and patented. For instance, U.S. Pat. Nos. 3,849,609 and 4,300,023 describe SLICs which are suited for manufacture in monolithic integrated circuit form. These prior art SLICs provide the desired interface between the telephone carrier equipment and the subscriber telephone handset while replacing bulky hybrid transformers.
A typical integrated SLIC circuit must provide both DC and AC impedance matching to the subscriber loop termination. Generally, impedance matching is established in a SLIC by using negative feedback wherein a pair of sensing resistors, having quite large resistance values, are made to appear much smaller. U.S. Pat. No. 4,300,023, mentioned above, describes a method of feedback utilized to perform this impedance matching.
SLICs are powered at the Central Office Exchange by a bank of batteries that supply an operating potential of -48 volts. In order to conserve battery power, while also reducing the power consumption in the SLIC, it is desirable to limit the maximum subscriber loop current supplied by the SLIC to a maximum value consistent with the operating requirements of the telephone handset. A problem arises due to the fact that the loop impedance presented between the SLIC and the telephone can vary between 100 to 1900 ohms depending on the distance the telephone is located from the Central Office. Typically, the loop current may vary from 20 milliamps (a subscriber loop of great distance) to 100 milliamps (a subscriber loop of very short distance).
Hence, a need exists for a SLIC of the type described above which can internally limit the value of the loop current supplied to the subscriber loop to a predetermined maximum value even though the SLIC is to be utilized in subscriber loop links which vary from very short distances to very large distances.